SURVEY OF SPECTRA

1. O-H 3600 N-H 3400 C-H 3000 C=N 2250 C=C 2150 C=O 1715 C=C 1650 C-O 1100 = = CARBONYL COMPOUNDS ( C=O BOND STRETCH) Aldehydes Ketones Esters Amides Acid Chlorides SURVEY OF SPECTRA

2. C=O STRETCHING

3. 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 Typical Infrared AbsorptionRegions C=O WAVELENGTH (mm) C-Cl C=O C=N O-H C-H C N Very few bands C-O C=C N-H C C C-N X=C=Y C-C * N=O N=O (C,O,N,S) FREQUENCY (cm-1)

4. THE CARBONYL STRETCHING REGION • This region stretches from about 1800 to 1650 cm-1 - RIGHT IN THE MIDDLE OF THE SPECTRUM • The base value is 1715 cm-1 (ketone) • The bands are very strong !!! due to the large C=O dipole moment. • C=O is often one of the strongest peaks • in the spectrum

5. 1715 C=O KETONE 2-Butanone BASE = 1715 overtone 2x C=O C-H CH bend C=O

6. BASE VALUE C=O IS SENSITIVE TO ITS ENVIRONMENT EACH DIFFERENT KIND OF C=O COMES AT A DIFFERENT FREQUENCY acid chloride carboxylic acid ester aldehyde ketone amide 1800 1735 1725 1715 1710 1690 anhydride 1810 and 1760 THESE VALUES ARE WORTH LEARNING all are +/- 10 cm-1 ( two peaks )

7. C=O BOND LENGTHS IN CARBONYL COMPOUNDS shorter longer 1.235 A 1.248 A 1.225 A 1.231 A acid chloride ester ketone amide 1780 cm-1 1735 cm-1 1715 cm-1 1680 cm-1

8. FACTORS THAT INFLUENCE THE C=O ABSORPTION

9. INDUCTIVE AND RESONANCE EFFECTS ON THE CARBONYL FREQUENCY O Electron-donating groups R C weaken the carbonyl and A lower its absorption frequency R = Me, Et, etc. O Electron-withdrawing groups B C X strengthen the carbonyl and raise its absorption frequency X = F, Cl, Br, O

10. INDUCTIVE AND RESONANCE EFFECTS ON THE CARBONYL FREQUENCY Resonance weakens the carbonyl and lowers its absorption frequency Y = N, O, or C=C C (Note the lengthening of the C=O bond! ) H O O Hydrogen bonding D lengthens and weakens the C=O bond and R C lowers its absorption frequency R

11. BASE VALUE HOW THE FACTORS AFFECT C=O STRETCHING VIBRATIONS acid chloride carboxylic acid ester aldehyde ketone amide B A C 1800 1735 1725 1715 1710 1690 D anhydride 1810 and 1760 ( two peaks ) A C E-donating Resonance B D E-withdrawing H-bonding

12. SUMMARY Ketones are at lower frequency than Aldehydes because of the second electron-donating alkyl group. Acid chlorides are at higher frequency than ketones because of the electron-withdrawing halide. Esters are at higher frequencies than ketones due to the electron-withdrawing oxygen atom. This is more important than resonance with the electron pair on the oxygen. Amides are at lower frequencies than ketones due to resonance involving the unshared pair on nitrogen. The electron-withdrawing effect of nitrogen is less important than the resonance. Note the electronegativity difference, O versus N, weights the two factors (resonance/ e-withdrawal) differently in esters than in amides. Acids are at lower frequency than ketones due to H-bonding.

13. CONFIRMING PEAKS

14. CONFIRMATION OF FUNCTIONAL GROUP Every type of carbonyl compound has other places you can look to confirm your conclusion based on frequency alone. C=O at 1710 cm-1 C=O at 1725 cm-1 also look for OH (H-bonded) and C-O ~1200 cm-1 also look for aldehyde CH 2850 and 2750 cm-1 C=O at 1735 cm-1 C=O at 1690 cm-1 also look for two C-O at 1200 and 1000 cm-1 also look for two NH peaks at 3400 cm-1 Ketones have C=O at 1715 cm-1 and no NH, OH, C-O or -CHO Anhydrides have two C=O peaks near 1800 cm-1 and two C-O

15. SELECTED SPECTRA

16. KETONE overtone of strong C=O peak 2-Butanone BASE = 1715 1719 x 2 = 3438 overtone C-H CH bend C=O 3438

17. ALDEHYDE Nonanal BASE = 1725 CHO >4C CH bend C=O 3460

18. ACID CHLORIDE Dodecanoyl Chloride BASE = 1800 CH bend C=O C-H 3608

19. ESTER Ethyl Butanoate BASE = 1735 C-O C-H C=O 3482

20. CARBOXYLIC ACID 2-Methylpropanoic Acid BASE = 1710 O-H C-O C-H C=O

21. RECALL CARBOXYLIC ACID DIMER lowers frequency of C=O and also of O-H Strong hydrogen-bonding in the dimer weakens the O-H and C=O bonds and leads to broad peaks at lower frequencies.

22. AMIDE BASE = 1690 two peaks sym / asym Propanamide C-H C=O NH2 CH bend